Supercapacitor Energy Storage for

MRI SystemsDr Mihailo Ristic

Imperial College LondonMechanical Engineering Department

London, United Kingdom

Outline:• Motivation: MRI power supply requirements• MRI principles, main system components• Imaging sequences and duty cycles• Reference MRI system analysis• Experimental and simulation studies• Novel MRI for magic angle imaging

Magnetic Resonance Imaging (MRI) Systems

Typical costs:• Scanner cost >1M€• Installation cost >1M€

Site preparation, room shielding, power supply• Running cost (incl. staff) ~0.5M€ per annum

MR Imaging modality:• Excellent soft-tissue contrast• Wide (and still increasing) range of contrast types

and diagnostic capabilities• Safe: no harmful ionising radiation (c.f, CT, PET etc.)• ‘Gold standard’ in diagnosing many illnesses

MRI magnet:• Typically superconducting• Temperature maintained at ~4K (liquid He)• Constantly running cryocooler

Mobile MRI UnitsTypically semi-permanent installations

• Space/capacity issues at hospitals• Temporary MRI capacity increase

On-board power unit:• Cryocooler & auxiliaries only• ~10kVA• Cannot support imaging

Typical on-site power requirementsGenerator Capacity Size Weight 200 kVA, 3-phase L5.0m x W2.5m 3.0 tons

200 kVA !!

• High power pulses• Role for energy storage?• Duty cycle considerations• Need to analyze functional details

MRI Principles• Our body is full of water H2O• 1H nuclei (protons) have magnetic spin:

• Little magnets • Placed in a strong magnetic field 𝐵𝐵0• Precess at Larmor frequency 𝑓𝑓0 (~64MHz @ 1.5T)

• External RF excitation flips the precession angle – resonance

Signal decay determined by

• Longitudinal relaxation time 𝑇𝑇1 and • Dephasing (transverse relaxation) time 𝑇𝑇2

• Resonant signal picked up by RF receiver coil

𝑇𝑇1and 𝑇𝑇2 are affected by tissue properties• Image contrast• Numerous mechanisms and weightings

3D image: Spatial encoding

• Main field is highly uniform (10−6T)• Gradients: controlled linear modulation

• Externally powered coils• 3 independent directions (X, Y, Z)• Field modulation → local modulation of frequency 𝑓𝑓0

• Spatial encoding directions:1. Slice select (Z)2. Frequency encode (X)3. Phase Encode (Y)

MR Image construction

• Data acquisition controlled by a pre-programmed imaging sequence

• RF excitation, • gradients switching, • data acquisition

• Raw data (k-space)• Volume slices• Voxels coded in frequency and phase

• Image formation: Fourier transform

RF

GSS

GPE

GFE

Signal

90° 90°

TR

Imaging Sequence

K-space dataImageFFT

Acqu

isitio

n

Imaging sequences• Huge variety• Duty cycle is hard to

predict / generalize• Rapid pulses (ms)

Gradient system • Resistive (copper) coils, integral with the main magnet

• High power pulsing:• High current (100’s A)

• High voltage (𝐿𝐿 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

= 100’𝑠𝑠 𝑉𝑉)

• Duty cycle = imaging sequence

Low Cost MRI• Novel asymmetric open magnet 0.5T

– good patient access• Low cost design:

• Single cryostat (bottom pole only)• Permanent magnet arrays for field

shaping• ‘Dry’ magnet (no liquid He)

Cryostat

Cryocooler

Permanent magnet

array

Power requirements:• Cryocooler: ~7.5 kVA continuous• Gradient system?

Gradient system specification

• Copley amplifiers (Analogic 266)

• Operating voltage range: 150V – 300V

Gradient CoilCurrent

(A)Voltage

(V)L

(μH)R

(mΩ)Rise time

(μs)

Slice select (Z) 220 150 160 22 120

Reading (X) 220 150 129 26.2 200

Phase encode (Y) 220 150 129 26.2 200

Gradient coil design

Nominal power requirementsGradients 99 kVA

Cryocooler ~8kVATOTAL >107 kVA

Our target: 10 kVA total• Is energy storage a feasible solution?• Recharge only between imaging

sessions?

Quick analysis: GRE sequence

Power profile

RF

GSS

GPE

GFE

Signal

90° 90°

TR

Pow

er (k

VA)

34.5

23.0

- 23.0

- 34.5

11.5

- 11.5

0

Time (milliseconds) 0 5 10 15 20 25 30 35 40 45 50

2

Without energy storage

With energy storage (target value)

Estimated frequency range:0 < 𝑓𝑓 < ~5 𝑘𝑘𝑘𝑘𝑘𝑘

Energy Storage Solutions

Batteries• Life: ~200 charge/discharge cycles

• ∴ Service life <6 months

Input

X-gradient coil

Y-gradient coil

Z-gradient coil

SupercapacitorBank

Power supply

Input Input

0

10

20

30

40

50

60

0.01 0.1 1 10 100 1000 10000

Capa

cita

nce

(F)

Frequency (Hz)

50 F cell BCAP-0050, Maxwell

5kHzSupercapacitors:• Long life: 106 cycles quoted• Compatibility with high pulse

frequencies?• Voltage variation – amplifier

compatibility?

Analysis method1. Specific application

• MRI system, supercap modules• Duty cycle: Full neurological examination:

• stroke protocol

2. Record accurate V,I waveforms• Run imaging sequences on MR spectrometer

3. Experiments (single cell): Equivalent circuit model • Electrochemical Impedance Spectroscopy (EIS)• Imaging sequence runs

4. Simulation studies• Full MRI examination

Candidate Supercap Modules

Analysis and experimentsSupercapacitor scaling:Equivalent 50 F constituent cell

Maxwell125 Volt Transportation Module 2 × in series = 250𝑉𝑉

3000 F

X 96

Equivalent Cell

250 V2.6 V

X 60

Experimental validation:• EIS• Imaging sequence (scaled)

Supercap model Equivalent circuit

Surface part - High-Freq

RaccLs R/n R/n

RleakC/n C/nCacc

R/n

C/n

Transmission line part: low-medium rangeof frequencies (0.01Hz – 3000 Hz)

Vscap

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

00 0.005 0.01 0.015 0.02 0.025

Z_im

agin

ary

(Ohm

)

Z_real (Ohm)

E = 2.2 V

E = 2.7 V

𝑹𝑹𝒂𝒂𝒂𝒂𝒂𝒂𝛀𝛀

𝑹𝑹 𝛀𝛀 𝑹𝑹𝒍𝒍𝒍𝒍𝒂𝒂𝒍𝒍𝐤𝐤𝛀𝛀

𝑪𝑪𝒂𝒂𝒂𝒂𝒂𝒂𝐅𝐅

𝑪𝑪𝐅𝐅

𝑳𝑳𝒔𝒔𝐧𝐧𝐧𝐧

EIS estimates 0.0129 0.03 36 0.91 55 2.7

Adjusted0.0109 0.0825 36 0.06 55 2.7

EIS test data (limited HF accuracy)

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

Time ( Seconds)

Cu

rre

nt (

Am

ps)

Spin Echo Current

SimulatedMeasured

0 0.005 0.01 0.015 0.02 0.0252.66

2.67

2.68

2.69

2.7

2.71

2.72

2.73

Spin Echo Voltage

Time (Seconds)

Vo

ltag

e(V

olts

)

SimulatedMeasured

Experimental fine-tuning(Spin Echo)

MRI SequencesWidely varying profiles and durations• Sequence type, parameters, # of slices,

2D vs 3D, etc.

RF

GSS

GPE

GFE

Signal

90° 90°

TR

Spin Echo90° 180°

RF

GSS

GPE

GFE

Signal

TE

TR

90° 180°

Gradient Echo

90° 180° RF

GSS

GPE

GFE

Signal

Echo-Planar (SE-EPI)

SequenceMatrix

sizeSlices NEX

TR (ms)

ACCTurbofactor

/ETLTE

(ms)

T1 Spin Echo 256 x 256 17 1 550 1 1 8.4

T1 Spin Echo 256 x 256 17 1 550 1 1 8.4

FLAIR 256 x 256 17 1 9000 1 16 89

T2 Spin Echo 512 x 512 17 2 5000 1 16 93Trace

diffusion (DWI)

192 x 192 17 1 3000 2 192 89

Time-of-Flight

Angiography256 x 256 80 1 23 1 1 7

SWI Volume scan

256 x 256 256 1 23 1 1 7

Stroke examination protocol (Hammersmith Hospital)

Focussed on stroke examination:• Representative of demanding MRI examination• Conducted extensive simulation studies using

verified models

Results:

0

50

100

150

200

250

300

0%20%40%60%80%100%

Supe

rcap

acito

r Ban

k Vo

ltage

(V)

Energy remaining in the supercapacitor banks (%)

SWI-3D

TOF Angio

DWI

SE T1SE T1

FLAIRSE T2

Gradient Amp power supply

Voltage vs. Stored Energy

Conclusion:1. Capable of accommodating high frequencies found in MRI2. Stored energy sufficient to conduct complete full neurological protocol3. Voltage range compatible with commercial amplifiers – simple topology

Total energy used:~585kJ

Assuming 2kVA supply• Recharging time ~7 min• vs. total examination

time ~30 min

Recent ImplementationYaw Motor

Roll Motor

Magnet

B0

• Novel MRI prototype, dedicated for imaging of the knee

• Unusual magnet configuration – transverse field

• Demanding gradient design → high power

GA -300 Maxwell BMOD0006

Gradient amplifiers:• 3 x Performance Controls, GA -300

300Volt, 200Amp• Recommended Power supply: 10kVA,

3- phaseSolution:

• In-series, 2 × Maxwell 160V, 5.8F modules

• Conventional 13A, single-phase socket

Magic Angle MRI System

Test phantom (0.5mm resolution)

Angle sensitive MRI: collagen fibre tractography

Manufactured system

Thanks to:• John McGinley• Vladimir Yuffit• Yann Gryska